That is the old, classic Garrett Seahunter case. I think Anchor Electronics had an u/w pulse detector called the Barracuda that looked similar and Eric Foster made several u/w pulse models using that case as well. It was very popular in the 80's and 90's and many people utilized it for their own homemade detectors.
Harold Holden had a couple of "clear case" detectors like that (yes like the Garrett case)
he was using in 90s...
think he got them from John Brandon and they were made in the Keys
agree the case could have been similar to a lot of put together units
I bought one like that years ago at kellyco.mine was a prototype that they made.No labels or name on it.Unfortunatly i loaded it to a so called friend and he never returned it.It was a copy of a garrett.
Thats the old "KW-1000" sold by Salvors Inc. I bought mine from Mel Fisher in 1994. He had his electronics guy (I think his name was Michele) building them and selling them at the Key West museum. They are excellent units. Very sensitive and easy to operate with just one knob.